The invention is directed to an apparatus for the measurement of the velocity of flow of gases and liquids, particularly the intake air of internal combustion engines, with one or more temperature dependent electrical resistances, which are in heat contact with the flowing medium, whereby one or more of the electrical resistances are heated with the help of a control circuit and the heated resistance or resistances are film resistances, consisting essentially of or consisting of a thin metal layer on an electrically insulating carrier.
In the interest of maintaining the purity of the air it is necessary to reduce the toxic materials produced by internal combustion engines, especially from automobiles. This object is attained inter alia by an improvement of the course of combustion. For this purpose it is necessary that the most exact information be available as to the amounts of air taken in in each case, namely with a smallest possible delay in time.
There have already been proposed hot wire anemometers of various constructions for the measurement of the amount of air taken in by internal combustion engines. The decisive disadvantage of customary hot wire anemometers is that dirt particles entrained in the air sucked in deposit on the heated resistance and through this change the gauging characteristics of the emission of heat.
In Eiermann U.S. application Ser. No. 304,052, filed Sept. 21, 1981, the entire disclosure of which is hereby incorporated by reference and relied upon), it was taught to use a metal film resistance which has the form of a plate which is aligned parallel to the direction of flow and the narrow side turned to the flow is coated with a heat insulating protective layer. Through this construction the influence of the dirt can be suppressed in satisfactory manner.
The electrical resistance of such a metal film resistance is advantageously brought to a desired value by producing cuts in the metal layer with a laser so that there is formed a meandering flow path. Customarily in such metal film resistances the geometry of the resistance paths is so chosen that the resistance produced per unit of surface area is distributed nearly equally over the entire resistance surface.
The response times producible with such a metal film resistance with sudden change of the flow velocity, however, for the most part are not sufficient to fulfill the necessary requirements for the control of an internal combustion engine. It is known that the response times of such an anemometer with heated metal film resistance are the shorter the smaller the thickness of the carrier material. However, this cannot be reduced arbitrarily since, based on the structure, the heated resistance must be self supporting and sufficiently mechanically stable.
Therefore it was the problem of the present invention to develop an apparatus for the measurement of the flow velocity of gases and liquids, especially the intake air of internal combustion engines with one or more temperature dependent electrical resistances which are in heat contact with the flowing medium whereby one or more of the electrical resistances is heated with the help of a control circuit and in which the heated resistance or resistances are film resistances consisting essentially of or consisting of a thin metal layer on an electrically insulating carrier of the type that the dirtying effect is suppressed to a satisfactory extent and there is attained a shortest possible response time with change of the air velocity.